1. Field of the Invention
The present invention relates to tuner circuits, and particularly to a tuner circuit for a cable television (CATV) set.
2. Description of the Related Art
A tuner circuit for a CATV set, similarly to a tuner circuit 1 according to the present invention, shown in a block diagram in FIG. 1, includes a variable bandpass filter (BPF) 5 that functions as an input tuning circuit for tuning to the vicinity of a signal of a desired channel among signals of a large number of channels, a variable-gain amplifier circuit 6 for amplifying or attenuating the level of a signal output from the variable BPF 5 to a prescribed level, and a narrow-band variable BPF 7 for tuning to a signal of a desired channel in an even narrower range among a signal output from the variable-gain amplifier circuit 6.
The variable-gain amplifier circuit 6 operates such that the gain thereof is reduced in accordance with an automatic-gain-control (AGC) voltage with a desired gain as a reference point. Thus, although the overall operation of the variable-gain amplifier circuit 6 is amplification, since the gain changes in accordance with the AGC voltage in a direction of attenuation with respect to the reference point, the change in gain will be expressed as “attenuation”. For example, the AGC voltage is set based on a level of a signal demodulated in a demodulation circuit disposed downstream of the tuner circuit such that the level of the demodulated signal is constant. The level of the demodulated signal is detected, for example, by an AGC detection circuit disposed downstream of the demodulation circuit.
With the above-described components alone, return loss (RL) of the tuner circuit 1 with regard to the variable BPF 5 and thereafter as viewed from the input terminal 2 is large (reflection is small) at frequencies of a signal of a desired channel, and return loss is small (reflection is large) at other frequencies. Since tuner circuits for CATV sets are connected to a common cable, if a channel having a large reflection exists as described above, a characteristic impedance mismatch of the cable occurs at that channel which could cause negative effects on, for example, the operation of a tuner circuit for a CATV set of another home. Thus, to improve return loss characteristics over the entire channel bands, a wide-band pre-amplifier circuit 4 that functions as a buffer amplifier and a low-noise amplifier is provided. The pre-amplifier circuit 4 also improves isolation such that a local oscillation signal is prevented from leaking from a mixer circuit included in the tuner circuit 1 to the cable via the input terminal 2.
The level of an RF signal input to a tuner circuit for a CATV set varies considerably; for example, about −15 to about +15 dBmV according to U.S. specifications and about −7 to about +25 dBmV according to Japanese specifications. Thus, distortion often occurs in the pre-amplifier circuit 4 or in the variable-gain amplifier circuit 6 when the signal level is high. To prevent distortion, a variable attenuator circuit 3 including a PIN diode connected in series with the signal path is provided at an upstream side of the pre-amplifier circuit 4, and the attenuation in the variable attenuator circuit 3 is controlled according to the same AGC voltage used to control the variable-gain amplifier circuit 6, thereby inhibiting a signal at a high level from entering the pre-amplifier circuit 4.
A tuner circuit configured as described above is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 10-276109.
Japanese Unexamined Patent Application Publication No. 10-276109 discloses a tuner circuit in which only a variable-gain amplifier functions as an attenuator to achieve the overall AGC operation while the level of an input RF signal is small, and in which a variable attenuator circuit starts to function as an attenuator only when the level of the RF signal has become large. Japanese Unexamined Patent Application Publication No. 10-276109 discloses that the above-described arrangement maintains a favorable noise factor (NF) while the level of the RF signal is small and also suppresses distortion in the variable-gain amplifier circuit when the level of the RF signal is large.
In the tuner circuit disclosed in Japanese Unexamined Patent Application Publication No. 10-276109, attenuation is achieved primarily by the variable-gain amplifier circuit, such that the attenuation in the variable attenuator circuit does not substantially increase even when the level of an input RF signal is considerably large. This means that a signal at a high level passes through the variable attenuator circuit without being substantially attenuated.
Regarding the variable attenuator circuit including the PIN diode disposed in series with the signal path, FIGS. 2A and 2B show the relationship between the level of intermodulation distortion and the level of an output signal corresponding to a direct current that flows through the PIN diode where the level of an input signal is about 10 dBm. FIG. 2A shows second-order intermodulation distortion (IM2), and FIG. 2B shows third-order intermodulation distortion (IM3). Since the level of the input signal is constant, FIGS. 2A and 2B indicate that the attenuation decreases as the level of the output signal increases. As will be understood from FIGS. 2A and 2B, the attenuation is substantially zero when the direct current is approximately 1 mA or greater, such that in actual operation, the direct current is maintained at no more than approximately 0.5 mA. The intermodulation distortion is small when the direct current is large and the attenuation is small; it increases as the direct current decreases and the attenuation increases, and reaches a peak when the direct current reaches a certain value; and it rapidly decreases when the direct current is approximately 100 μA or less and the attenuation continues to increase.
When a positive-intrinsic-negative (PIN) diode that functions as a variable attenuator circuit is connected in series with the signal path as described above, while the attenuation is relatively small, intermodulation distortion increases as the attenuation increases.
Furthermore, since the attenuation in the variable attenuator circuit is small and a signal at a high level passes through the variable attenuator circuit, a signal at a high level is input to the pre-amplifier circuit. Thus, distortion inevitably occurs in the pre-amplifier circuit.